Zinc Excess Impact on Growth and Nutrition of Trigonella foenum-graecum L. under Symbiotic Nitrogen Fixation and Nitrogen Fertilisation Conditions

Accumulation of Zinc in soil at high concentration leads to drastic changes in the structure of microbial communities and a decrease in soil fertility. Legumes growing on Zn-contaminated soils are negatively affected. Based on our previous work, Trigonella foenum-graecum L. was cultivated on a chosen concentration of zinc (0.75 mM ZnSO4) under three different regimes of nitrogen sources, without nitrogen, nitrogen fertilisation (with urea) or nitrogen symbiotic fixation (inoculated with specific rhizobia). Results showed a significant zinc effect on shoot dry weight with a reduction more pronounced in symbiotic regime (36.9%). Concerning roots, plants showed only a reduction of 12.6% in nitrogen fertilisation regime against 31% and 58.4% with zero nitrogen and symbiotic nitrogen fixation regime respectively. Moreover, number and dry weight of nodules were significantly affected. The supply of exogenous zinc (0.75 mM) induced a significant increase of this metal in shoot for plants without nitrogen (0.6 mg/g DW) and plants with nitrogen fertilisation (0.26 mg/g DW). The notable result was the increase of zinc content in root and nodules of plants inoculated with rhizobia under zinc supply. Zinc showed a significant increase of Ca content in shoots and roots in inoculated plants. In conclusion, the effect of zinc on T. foenum-graecum is variable according to the nitrogen source with superiority in growth parameters for nitrogen fertilisation and preference for symbiotic nitrogen fixation for phytostabilisation of zinc and translocation and uptake of nutrients.